Recent years have witnessed widespread use of mobile phones and PDAs (Personal Digital Assistants). To respond to growing demand, a lot of efforts are put into the development of displays capable of on-screen information presentation mounted in these electronics.
Displays are classified into two major categories: non-light-emitting and light-emitting. The former incorporates a light-modulating element modulating light from an external light source, such as sunlight, indoor lighting, backlight, or frontlight, to produce a display. A typical, well-known example is the liquid crystal display element. The latter needs no external light source and the light-emitting element emits light on its own to produce a display. A typical example is EL (Electro Luminescence), which is attracting a lot of interest. The following will describe these types of displays in more detail.
First, the transmissive-type liquid crystal display which is a non-light-emitting display using an external light source uses a backlight as the light source and is therefore energy-consuming and bulky, which poses problems for mobile uses. To address one of the problems, in other words, to reduce large power consumption, the reflective-type liquid crystal display has been developed in which the lower electrodes of the liquid crystal layer is made of aluminum or another light-reflecting metal, so as to use external lighting, such as sunlight indoor lighting, as a light source. However, the reflective-type liquid crystal display has a shortcoming: since its operation relies on external lighting, it cannot be used under poor lighting.
To address these problems, the transflective-type display has been developed which includes lower electrodes of the liquid crystal layer fabricated from a half mirror. The display effects a reflective-type display without using a backlight under sufficient lighting and a transmissive-type display with the backlight turned on under poor lighting. The transflective-type display, however, has a low light using efficiency and makes a poor candidate to lower power consumption, because it relies on the conflicting characteristics of the light-reflecting part and the light-transmitting part for operation.
To address these problems, the inventors invented a liquid crystal display (see U.S. Pat. No. 6,195,140 B1; Date of Patent, Feb. 27, 2001) which acts as a reflective type without using a backlight under sufficient lighting and a transmissive type with the backlight turned on under poor lighting.
Unlike conventional liquid crystal displays incorporating a reflector which is so thin to acquire transflectivity, the liquid crystal display has each display pixel divided into two areas: a reflective area and a transmissive area. Specifically, in the liquid crystal display, a reflective electrode is provided in one of the areas of each display pixel, forming a reflective area. In the other area is provided a transmissive electrode, forming a transmissive area. Besides, the liquid crystal layer differs in thickness between the reflective area and the transmissive area. The configuration optimizes luminance both in the reflective area and in the transmissive area.
A drawback of the liquid crystal display of pixel-dividing type is low efficiency in backlight's light usage, because the backlight's light is projected from behind onto all the pixels, i.e. both of the two areas, but is used only in the transmissive area. The higher percentage the reflective electrode accounts for, the smaller the transmissive area and the lower the efficiency in using the backlight's light. This is inevitable.
Accordingly, the low light using efficiency problem in the pixel-dividing type liquid crystal display is addressed in, for example, Tokukai or Japanese Laid-open Patent Application 2001-66593 published on Mar. 16, 2001. The application discloses a pixel-dividing type liquid crystal display (labeled 300 in FIG. 41) has through holes 304 in part of reflective electrodes 303 of pixels 302 on a liquid crystal panel 301. The liquid crystal display 300 includes a light-emitting element, which is an organic EL (Electro Luminescence) element 310, constituting a backlight. These light-emitting portions 311 in the organic EL element 310 are not provided across the entire area of each pixel 302, but only in those parts where the through holes 304 are provided. Incorporating a patterned organic EL element as a backlight in this manner allows for improved light using efficiency and reduced power consumption.
A display incorporating an organic EL element, which is a typical light-emitting display, is advantageously thin and lightweight. The self-illumination enables the display to be, unlike liquid crystal displays, visible under poor lighting without a need for a backlight. Outgoing light is used almost solely for the purpose of display, achieving a high efficiency in light usage. The display incorporating an organic EL element however needs constant light emission and to improve on the display quality, requires increased light emission, which are obstacles in reducing power consumption.
Since in the pixel-dividing type liquid crystal display shown in FIG. 41, the organic EL element 310 as a light-emitting element is placed outside the liquid crystal panel 301, there are interposed a retardation plate 305, a polarizer 306, and two glass substrates 307, 312 between the organic EL element 310 and the through holes 304 in the reflective electrodes 303. The through hole 304 has a width ½ to ⅙ the pixel pitch, and given a currently typical pixel pitch of approximately 80 μm, the through hole 304 is about 15 μm to 40 μm in width. The polarizer 306 has a thickness of about 300 μm. The two glass plates, or the glass substrate 307 of the liquid crystal panel 301 and the glass substrate 312 of the organic EL element 310, has a combined thickness of 500 μm to 700 μm. All together, the organic EL element 310 is separated from the through holes 304 in the reflective electrodes 303 by a distance as large as 1300 μm to 1700 μm. It is therefore impossible to guide the whole of the outgoing light from the light-emitting portions 311 of the organic EL element 310 so as to enter the through holes 304, although the light-emitting portions 311 of the organic EL element 310 are placed at positions corresponding to those of the through holes 304. The problem of low efficiency in light projection by the organic EL element 310 still remains unsolved.
In any case, no change is made to the fact that the pixel-dividing type liquid crystal display in FIG. 41 includes two substrates which are stacked one on the other, and its thickness cannot be reduced further below the total thickness of the liquid crystal display and the organic EL element. Moreover, in the structure shown in FIG. 41, the through holes 304 of the liquid crystal display and the organic EL element 310 need be precisely positioned and combined in manufacture. The procedure requires a dedicated positioning device and combining mechanism, which increases component counts and manufacturing cost.
As mentioned in the foregoing, the reflective-type liquid crystal display has been developed for improved outdoor visibility, delivering superior visibility outdoors under strong external light, but the display is unusable indoors and at night. Proposed as a substitute for the external light is the reflective-type liquid crystal display which employs frontlight illuminating the device from its front. For example, Tokukai or Japanese Laid-open Patent Application 2000-75287 published on Mar. 14, 2000 discloses an organic EL element used as a frontlight. However, the same problem arises as in the foregoing case where a backlight is incorporated in the transmissive-type liquid crystal display: the total thickness is too large due to the contribution from both the display and the supplemental light source.
Forming a liquid crystal display element and an organic EL element on a single substrate as in the foregoing offers a viable solution to supplement each element's disadvantages and achieve an optimal display in various environments.
Nevertheless, simply forming a liquid crystal display element and an organic EL element on a single substrate in the foregoing display makes wiring and drive circuits in the substrate too complex, entailing problematic low yields and high costs in manufacture.
There are other problems in the manufacture of a light-emitting display element containing an organic EL element as a light-emitting element.
For example, Tokukai or Japanese Laid-open Patent Application 2000-173770 (published on Jun. 23, 2000) discloses a method of forming a TFT (Thin Film Transistor) circuit driving an organic EL element on one of two substrates and metallic electrodes which will become cathodes and a part of an organic layer constituting an organic EL layer on the circuit, forming anodes on the other substrate and a light-emitting layer on the electrodes, combining the two substrates under applied heat or pressure to heat an organic layer beyond the glass's transition temperature so that the two substrates are securely attached to each other.
Tokukai or Japanese Laid-open Patent Application 2001-43980 (published on Feb. 16, 2001) discloses a method of forming anodes on a substrate (TFT or else) and sequentially adding thereon a hole introducing layer which will be an organic EL layer, a hole transport layer, and a light-emitting layer, which is then followed by the formation of a thin film of metal having a low work function which will be a cathode and the formation of a transparent conductive layer.
In both disclosures, the outgoing light from the organic EL element may leave not the side facing the substrate on which a circuit is formed to drive the organic EL element, but the side facing an opposite substrate oppositely placed to that driver-carrying substrate or a protective layer. Thus, the outgoing light will less likely be blocked by circuit patterns than the outgoing light from the circuit-carrying side, which achieves an increased aperture ratio and is effective in improving on luminance, light-emitting efficiency, life, and reliability.
On the drive circuit-carrying side, the area conventionally reserved for hole formation is now used for circuit. Circuit designs can be more flexible, contributing to improvement of reliability and yield, and the method is effective in enabling circuit formation with improved function. Specifically, it is realized by separately forming the drive circuit side and the light-emitting layer side in Tokukai 2000-173770 and by forming extremely thin cathodes in Tokukai 2001-43980.
Under this circumstance, among other materials, no water is preferably allowed to enter the organic EL element for the sake of reliability of light-emitting function. The organic conductor, if oxidized, can degrade due to acceptor doping. The metal making up the cathodes is magnesium (Mg), lithium (Li), calcium (Ca), or other materials with a low work function and susceptible to oxidation and difficult to work on.
As detailed above, the organic EL element has following features: it has a simple structure, but is made of materials of which functions are readily affected by the surroundings. In the fabrication an organic EL element, it is preferred if the element is completely formed in an environment which is free from water and oxygen as much as possible and the layer providing protection to the light-emitting layer is formed at the same time.
Regarding this issue, Tokukai 2000-173770 securely attaches at a part of the organic layer constituting an organic EL element and likely allows contact with atmosphere containing water and oxygen in an attaching process; reliability is a problem. Each organic layer making up the organic EL element is so thin as about 1000 Å that the layers can lose uniformity in quality and function when combined, in a process to form a part of them on the respective substrates and heating to a temperature beyond glass's transition point.
Tokukai 2001-43980 places metallic cathodes on a light-emitting side and however thin the cathodes, suffers resultant transmission loss. The cathodes have setbacks due to its extra thickness: it may degrade because of bonding with a transparent conductive layer and an organic conductive layer formed on it; temperatures in the formation of the transparent conductive layer may negatively affect the light-emitting layer.
Tokukai 2000-173770 employs a transparent conductive film on the anode side and therefore exhibits a greater resistance than an ordinary conductor. When incorporated in a panel, the transparent conductive film develops electric energy loss and develops spots to appear on screen where luminance alters.